In the answer to question no.
46544 and
32577 we have
explained that the time for Witr is from after ‘Isha’ prayer until dawn
breaks.

If the muezzin gives the adhaan for Fajr whilst you are
praying Witr, then Shaykh Ibn Baaz (may Allaah have mercy on him) said:

What is prescribed for every believing man and woman is to
pray Witr every night, the time of which lasts from ‘Isha’ prayer until dawn
breaks, because it is proven in al-Saheehayn from Ibn ‘Umar (may
Allaah be pleased with him) that the Prophet (peace and blessings of
Allaah be upon him) said: “The night prayers are two by two, then when you
fear that dawn is about to break, pray one ra’kah to make the number of
rak’ahs that you have prayed odd.”

There are many ahaadeeth which speak of this. This indicates
that Witr ends when dawn breaks. If a worshipper does not know when dawn
breaks, he may rely on the muezzin who is known to seek the correct time.
When the muezzin who is known to seek the correct time gives the adhaan,
then the time for Witr has ended. But if he gives the adhaan before dawn
then the time for Witr does not end when he gives the adhaan, and that does
not mean that it has become haraam for one who is fasting to eat and drink,
or that the time for Fajr prayer has begun.

From what we have mentioned, it is clear that the time for
Witr ends with the first adhaan if the muezzin is seeking the correct time
of dawn for his adhaan. But if the muezzin gives the adhaan when the Muslim
is in the last rak’ah, he should complete it because he cannot be certain
that dawn has broken just from hearing the adhaan, and there is nothing
wrong with that in sha Allaah. End quote.

Majmoo’ Fataawa Ibn Baaz
(11/305)

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was
asked about a man who is praying Witr and whilst he is praying the muezzin
gives the adhaan for Fajr prayer. Should he complete his prayer?

He replied: Yes, if the adhaan is given whilst he is praying
Witr, he should complete his prayer and there is nothing wrong with that.
End quote.